Frequency modulation



NOV. 30, 1948. v, LANGMUIR 2,454,971

FREQUENCY MODULATION Filed ma 22, 1946 /2 a M11 /44 33 ,2 l I IVfGAf/Vf RIS/Sfl/VCI 7V 44x47: ezs/smlvcz pownasowa: A2 7&2 POM/7? sol/R05 Fig. 2. 4 Fig. 4.

A M A 1 kzbv It 26 2/ I ZQ Q OSCILLATUR USC/LLATOI? Inventor: Robert V. Langmuir,

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His Attorney name s me, 1948 i v 2,454,971

FREQUENCY MODULATION Robert V. Langmuir, Schenectady, N. Y.', assignor to General Electric Company, a corporation of New York Application May 22, 1946, Serial No. 671,480

4 Claims. (Cl. 332-5) UNITED STATES PATENT OFFICE 2 My invention relates to methods and apparatus invention; Fig. 5 represents schematically a cirfor controlling the frequency of high frequency cult embodying the principles of the invention in oscillators and more particularly to a method and connection with oscillators of the type employing apparatus which'is especially useful for effecting electron discharge tubes of the non-magnetron frequency modulation and particularly so with 5 type; while Fig. 6 represents schematically an oscillators of the magnetron type. embodiment of the invention employing masne- As is well known in the art of electrical comtron type oscillators. munications, the frequency modulated oscillator Generally speaking, I accomplish the objects of constitutes a highly useful tool for the trans- I the invention by mutually coupling together two mission of intelligence by frequency modulation 10 oscillating elements which would have different methods, 1. e., methods in which continuous varinatural frequencies if isolated and permitted to ations in the oscillation frequency from a standoscillate independently without any mutual couard frequency constitute'a decipherable signal pling between each other. The mutual coupling corresponding to the intelligence to be transis accomplished in such fashion that the intermitted. To be'of optimum practical value such action of the electromagnetic fields of the an oscillator must be capable of rapid, continuous elements forces them to oscillate at a common and simple adjustment of its frequency throughfrequency which may be caused to vary throughout a desired frequency spectrum and must be out a substantial frequency spectrum by varying free of distorting factors such as undesirable varithe relative amounts of power supplied to the ations in amplitude (amplitude modulation) individual elements. which in certain types of oscillators frequently The general principle of the invention will accompany variations in frequency. The usefulbest be understood from a consideration of ness of the magnetron, as a source of oscillations the behavior of the coupled circuit shown for communication and related purposes has herein Fig. 1. In that figure Ill and ii repretofore been limited by inherent characteristics sent coupled circuits having slightly diflerent which render difllcult frequency modulation natural resonant frequencies corresponding to meeting this standard. wave lengths M and a, respectively. The circuits I have discovered a novel method combining may be considered as mutually coupled by a two oscillating elements into a single system in mutual inductance M. It will be understood that such a manner that readily controllable variaif some convenient source of power having negations in a common frequency of the elements may tive resistance characteristics be added to the be effected and used as desired for the purpose of combined circuit free oscillations may be set up general frequency control including the transtherein. Such a source of power is indicated mission of intelligence by frequency modulation schematically as power source I2. The combinamethods. While the method is applicable to oscil- 5 tion will, however, oscillate at a comon wave lators of any type it is particularly useful in conlength A differing from both A; and M. A connection with magnetrons in view of the aforesideration of the theory of coupled circuits (of. mentioned dificulties heretofore experienced in "Theory of Thermionic Vacuum Tubes" by E. L. modulating magnetrons. Accordingly, it is the Chaifee, first edition (1933, page 378) will indigeneral object of my invention to provide a novel gate th t th value of r for given value-of M method and apparatus capable generating high can be shown to be a function of the ratio M/M frequency Oscillations of Variable which function is best illustrated by the curve of frequency suitable for frequency modulated trans- Fig 2 showing the ratio A1 as ordinates and the mission of intelligence.

The features of the invention desired to be pro- 5 Zi of as absclssae' The precise form of tected herein are pointed out m the appended e curve will vary greatly with different values claims. The invention itself, together with its M but that shown-1n 2 may be taken as a further objects and advantages, may best be um qualitative representation of a form of curve derstood by reference to the following description which may be Obtained by appmprlate selection of taken in connection with the appended drawing 0 the coupling AS hereinafter explained in in which Figs. 1 and 3 represent schematically greater detail, the curve will assume undesirable coupled circuits illustrating the general principles shapes for extreme values of coupling. for of the invention; Figs. 2 and 4 represent graphs very tight couplings and for very loose couplings. of certain characteristics of the circuits of Figs. Sufllce it to state at this point that intermediate 1 and 3 useful in explaining the principle of the values of M such as will cause the curve to assume the general shape indicated in Fig. 2 are preferable.

Assume now for purposes of illustration that M and M are given fixed values such that M M (A: is greater than )u) i. e., 7\2/?\l 1 (la/x1 is greater than 1) If then the sytem is permitted to oscillate freely under the stimulation from the negative resistance power source II, the resultant wave length of free oscillation M will be indicated by some point a on the curve of Fig. 2 from which it becomes apparent that At ii (A. is less than A1).

Consider next the situation which obtains if the negative resistance power source i2, as indicated in Fig. 3, is transposed to the circuit l'l with all the circuit parameters of both circuits remaining unchanged. A curve similar to Fig. 2 may now be drawn showing the relation of the ratio A/Az to the ratio M/M and such is indicated by Fig. 4. Since the circuit parameters are the same as those discussed in connection with Fig. 1, i. e. M M it is obvious that As. the wave length of free oscillation for this case, will be indicated by some point b on the curve of Fig. 4 since \l/ \z 1. It follows that xb 7o and that is is greater than M. Thus by shifting the power from one circuit to the other a shift in the common frequency of the system has been efiected.

Now I have found that if the power be simultaneously applied to both circuits in varying proportions between the condition in which the power is proportionately all in the circuit l and that in which it is proportionately all in the circuit II it is possible to cause the frequency to shift continuously throughout a substantial range of frequencies between values corresponding to is and At. The analysis of the action in this situation is complicated since it involves phenomena of mutual interaction between the oscillators in circuits I0 and II causing them to lock in with each other at a'common frequency. It should suffice, however, to state that intermediate between the condition of very tight coupling and that of very loose coupling there may be chosen a value of coupling M at which continuous variation in A may be effected as the power input is gradually shifted from the circuit to the circuit H, or vice versa. If the coupling is very tight the frequency change will be accomplished as a discontinuous Jump from the one extreme to another. On the other hand, if it is too loose both circuits will oscillate independently at different wave lengths.

A complete circuit in which the foregoing principles may be embodied for the purposes of frequency modulation is shown in Fig. 5. In that figure two coupled oscillators i3 and H are mutually coupled by mutual inductance M between tuned tank circuits IE and i8 having different natural wave lengths when isolated from each other and connected respectively to oscillating triodes I1 and I8. As will be apparent from the drawing, each of the oscillators I3 and i4 is of the type in which a portion of the power from the oscillating plate tank circuit is inductively fed back to the respective grid circuit to sustain oscillations. Specifically in the case of oscillator IS, the oscillations are sustained in a well known manner in the tank circuit l5 comprising inductance I9 and variable capacitance by means of voltage imposed on the grid 2| by the lower portion 22 of the inductance l8 and the grid lead 23. A grid leak comprising the capacitance 24 and the resistance 25 is provided for the purpose of establishing a fixed bias on the grid in the conventional manner. The anode 26 of the triode i1 is connected to the tank circuit through a capacitance 21. A thermionic cathode 28 energized by any suitable heater (not shown) supplies the necessary electron currents in the known manner. Except for circuit constants determining its natural wavelength of oscillation, the oscillator l4 may be identical with the oscillator l3 although it need not necessarily be identical. For convenience of illustration it is shown as identical and comprised of corresponding elements which will be apparent by comparison with the oscillator l8. Both oscillators i 3 and I4 are energized by a source of anode voltage 29 connected to the respective anodes through secondary 30 of transformer 3i and radio frequency chokes 82 and 13, respectively. For the purpose of extracting energy from the system, a suitable output coil 34 inductively coupled to the tank circuits i5 and 18 may be provided. The coil 34 may be connected to an antenna (not shown) or other agency for utilizing the energy developed by the apparatus.

For the purpose of varying the amount of power supplied to the respective oscillators I3 and H in accordance with the principles of the invention, there is provided a modulator circuit indicated generally by the numeral 35. The modulator circuit 35 comprises a pair of amplifier triodes 38 and 31 having respectively anodes 38 and 39, grids 40 and 4|, and thermionic cathodes 42 and 43 energized by any suitable heater such as an electrical resistive heater (not shown). A suitable source of power 44 supplies energizing voltage to the anodes 38 and 39 through the medium of a center tapped transformer primary 45 of transformer 3i. As indicated. the secondary 8B of the transformer 31 is similarly center tapped and connected in the common anode circuits of the oscillators l3 and i4. Grids 40 and H are connected to the cathodes 42 and 43 through opposite halves of the center tapped secondary 46 of a transformer 41. The primary 48 oi the transformer 41 may be connected to any suitable source of modulating signals indicated schematically as oscillator 49.

In operation of the circuit of Fig. 5 modulator will serve as a push-pull amplifier which imposes an amplified modulating signal originating in the oscillator 49 upon the secondary 30. In the manner of push-pull amplifiers the transformer 31 will on one half cycle impose a maximum positive voltage on the anode 28 of oscillator i3 and a maximum negative voltage on the corresponding anode of the oscillator I4. During the alternate hali cycle, the process will be reversed such that the maximum negative voltage is applied to the oscillator 18 while the maximum positive voltage is applied to the oscillator l4. In this manner the oscillators i3 and I4 will be caused to oscillate with alternately varying amounts of power during the full cycle of the modulating voltage. During one half cycle the oscillator i3 will have a minimum amount of power while the oscillator I4 has a maximum, and during the next half cycle the situation will be reversed. In accordance with the principles of the invention heretofore discussed the oscillation frequency of the system will thereby be caused to fluctuate continuously between maximum and minimum values thereby effecting frequency modulation of the generated electromagnetic energy.

In the circuit of Fig. 6 there is shown a further embodiment of the principle of the invention in connection with oscillators of the magnetron type. In that circuit there are provided magnetrol oscilof the well known type may be developed. As

is well understood in the art. such waves will no developed by the appropriate imposition of operating voltages on the anodes of masnetrons U2 and as .througha suitable source of power ll connected between the respective cathodes s1 and II and the split' anodes B9 and Gil of the Y magnetrons l2 and M. as shown. It will be understood that the magnetrons will each operate in a magnetic field I-ldirected parallel to the axis of the cathodes M and B8 in the well known manner.

' wire systems M and ,w may be magnetically coupled to each other in a manner similar to that of the coils of Fig. i by being closely iuxtaposed to each other. However. the coupling may also be accomplished by means of a quarter wave interconnecting line Bi which may comprise a similar lecher wire system or the-equivalent. such as a coaxial cable of quarter wave lengthier the desired frequency of operation. (It will be understood that such an interconnecting line is roughly the equivalent of an actual inductance insofar as the reflection of impedance between the two circuits is concerned. I

For the purpose of varying the amountof power supplied to the respective magnetron oscillators l and Bi theremay be provided a modulator 82 similar. to that provided in the case or ig. 5. The modulatortt may comprise a pair ,oi ampliiler modes and M. having respectively, anodes 68 and 08. grids 8? and 88. and thermionic cathodes GI and iii which may be energized by any suitable source or heat such as an electrical resistive heater (not shown). A suitable source of power Ii supplies energizlng voltage to the anodes 8B and 88 through the medium of a center tapped transformer primary '72. As indicated, the secondary 8 of the transformer is similarly center tapped and respective halves connected in the anodecathode circuits of the oscillators 50 and M.

' Grids 81 and Bil are connected to the cathodes I! and I0 through opposite halves of the center tapped secondary ll of the transformer 15. The secondary It of the transformer It may be connected to any suitable source of modulating signals indicated schematically as oscillator II.

In operation it will be assumed that the parameters of the tank currents oftheoscillators' l0 and BI, 82 and 53 are so chosen that the oscillators' would tend to oscillate at somewhat ditferent i'requencies if isolated from each other with no mutual coupling. As is well known in the art. magnetron oscillators may be varied in frequency by suitable selection of such parameters as the length of the'lecher wire systems in the external circuits. Thus one way of etlecting such diiference in frequency would be to make one of the lecher wire circuits M and it slightly longer than the other. In a; manner similar to that discussed in connection with Fig. the secondary 18 will on one half "cycle impose a maximum positive voltage between the cathode and anode of magnetron .82 and a maximum negative voltage between thecorresponding cathode and anode generally as lii'and pi esoh'com- Y Such iieids maybe established by any suitable source (not shown! such-as a permanent with varying amounts of power-during the full cycle of the modulatin voltage. During one half cycle the oscillator Bil will have a minimum amount of power while the oscillator Bi will'have situation will be reversed. In accordance with the principles of the invention heretofore dis.

cussed,- the oscillation frequency of the system will thereby be caused to fluctuate continuously 7 between maximum and minimum values thereby effecting frequency modulation .of the electromagnetic waves. j It will be obvious that the principles of the invention may be employed not only for the purposes of the transmissionof intelligence by frequency modulation methods but also for frequency control purposes in general. For example. it may be used for tuning purposes where it is .desired to tune an oscillating system to a. fixed frequency selectable at will within a given range olfrequencies. In such case, the relative amounts of power in the respective oscillator heretofore described may be fixed by any suitable means at such values as produce the desired ilxed frequency. I a

' It will also be understood that the principles of Fig. 6 may be applied to any type magnetron now common in' the art such as the cavity resonator type or the vane type in which .oscillav tions are generated in. space resonant cavities positioned about the periphery of a rotating elecover, any of the known types of anode tank cir-' cuits may be used in the place of either the lecher wire circuits M and 85 or tank circuits It; for example. coaxial cable sections of appropriate fractional or nrultiple wave lengths, or the tank circuits shown in Flgs fi and 6 may beinterchanged between those figures to be used with either pf the types of tubes there shown. It will beund'erstood as well that. although I have shown specific forms of mutual coupling between the oscillating circuits, the term mutual coupling is used in its broadest sense andmay include many other forms of coupling such as capacitative. resistive, or directly connected inductive cir- 05 to cover all such changes and modifications as invention.

What I claim as new and desire to secure by Letters Patent of the United Statedis;

1. Apparatus ior generating an alternating electrical potential of variable frequency comprising a pair of magnetron oscillators each. ad- Justed to oscillate at a difi'erent characteristic frequency when isolated from the other. each of 76 said oscillators including a magnetron and a .1 Q 6 v .of magnetron It. 'Durin'gIthe alternate half cycle the process will be reversed so that the maximum 4 a maximum ,and during the next half cycle the enerated fall within the true spirit and scope of my electrical potential of variable frequency com i prising a pair of oscillators. each adjusted to oscillate at a diiferent characteristic frequency when isolated from the other, mutual coupling means for coupling the electromagnetic fields of said oscillators whereby the mutual interaction of said fields causes said oscillators to oscillate at a common frequency, power means for imposing an energizing voltage on each of said oscillators,v and control means for varying the tinuously throughout a substantial range of frequencies, said control means including a source of alternating voltage and a push-pull amplifier for imposing said voltage upon said oscillators in push-pull manner whereby the voltage upon each of said oscillators varies between a maxi- 'mum and a minimum value during each cycle relative amounts of power in each of said oscillators whereby'said common frequency may be varied, said mutual coupling means having an intermediate value of mutual coupling such that said common frequency maybe caused to vary continuously throughout a substantial range of frequencies, said control means including a source of alternating voltage and a push-pull amplifier for imposing said voltage upon said oscillators in push-pull manner whereby the voltage upon each of said oscillators varies between a maximum and a minimum value during each cycle of said alternating voltage with substantially 180 degree electrical phase displacement between the voltages to each oscillator.

4. Apparatus for generating a frequency modulated electromagnetic wave comprising a pair of magnetron oscillators each having an anode, a cathode and a resonant transmission line operatively connected to said anode, said transmission ,lines being adjusted to cause said oscillators to oscillate at different characteristic frequencies when isolated from each other, mutual coupling means for coupling said transmission lines whereby the mutual interaction thereof causes said oscillators to oscillate at a common frequency, said means-coupling said transmission lines comprising a transmission line interconnected therebetween and having a length substantially onequarter of the average wave length of oscillation of said circuit, means imposing an energizing voltage between said anode and cathode in each of said oscillators to sustain oscillation at a of said alternating voltage with substantially 180 degree electrical phase displacement between th voltages upon each oscillator.

' 3. Apparatus for generating an alternating electrical potential of 'variable frequency comprising a pair of magnetron oscillators each adjusted to oscillate at a different characteristic frequency when isolated from the other, mutual coupling means for coupling the electromagnetic fields of said oscillators whereby the mutual interaction of said fields causes said oscillators to oscillate at a common frequency, power means for imposing an energizing voltage upon each of said oscillators, and control means forvarying the relative amounts of power in each of said oscillators whereby said common frequency may be varied, said mutual coupling means having an intermediate value of mutual coupling such that said common frequency may be caused to vary concommon frequency, and means for varying the relative amounts of -=said energizing voltage in each of said oscillators whereby said common frequency may be varied, said last mentioned means comprising a source of alternating modulating voltage, means foriimposing said modulating voltage between said anode and cathode in each of said oscillators in push-pull manner whereby said energizing voltage in each of said oscillators varies between a maximum and a minimum value during each cycle of said alternating modulating voltage with"-substantially 180 degrees of electrical phase displacement between the variations in the said energizing voltages in each said oscillator.

ROBERT V. LANGMUIR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Usselman Dec. 8, 1942 

